It is well known that chlorine dioxide gas is a strong oxidant, and its oxidizing action is effective in sterilization and decomposition of malodorous substances. Therefore, chlorine dioxide has been used in disinfectant, deodorant and the like. Chlorine dioxide is dissolved in water in 20 times its volume of water, to give a brownish yellow aqueous solution. From the viewpoint of easiness in handling, it is desirable to use chlorine dioxide in a form of such an aqueous solution. However, when the aqueous solution of chlorine dioxide is brought into contact with air, chlorine dioxide gas is rapidly generated. Therefore, there has been proposed a technique in which chlorine dioxide gas is constantly generated while maintaining its stability, by dissolving chlorine dioxide gas in an aqueous solution of sodium peroxycarbonate, and thus by forming an aqueous solution containing sodium chlorite as a main component at a retained pH of 9, i.e., what is called a stabilized aqueous solution of chlorine dioxide (see Patent Document 1).
The stabilized aqueous solution of chlorine dioxide is retained at pH 9 (alkali) for the purpose of maintaining stability, as described above. Therefore, a generation amount of free chlorine dioxide gas having disinfecting and deodorizing effects or the like is extremely low, and thus it is difficult to attain satisfactory disinfecting and deodorizing effects or the like. Therefore, in order to enhance the activity of the stabilized aqueous solution of chlorine dioxide, an acid has been added immediately before its use to lower the pH to 7 or less, for generating chlorine dioxide gas, to thereby elicit activity.
However, the generation of chlorine dioxide gas is extremely rapid, and chlorine dioxide does not have sustained activity. As a result, there arise economical problems that processes and equipments or facilities to implement the processes are required in order to activate the chlorine dioxide solution and to continuously generate chlorine dioxide gas. In addition, since chlorine dioxide gas is rapidly generated in the conventional stabilized aqueous solution of chlorine dioxide as described above, a chlorine dioxide activity is not retained constant and sometimes reaches an extremely high level, which raises safety concern about effects on animals, especially on human being, and the chlorine dioxide has not been used without anxiety.
In order to solve the above-mentioned problems, there has been proposed a technique in which a mixture prepared by adding an organic acid, such as citric acid, to chlorite is blended with a dissolved chlorine dioxide solution, to thereby maintain a chlorine dioxide concentration nearly constant for a long term (see Patent Document 2).    Patent Document 1: Japanese Patent Application JP61-181532A    Patent Document 2: Japanese Patent JP3110724B